Optimized control of a heater for an air dryer

Abstract

An air dryer having a heater associated with its inlet and exhaust valves to prevent freezing at cold temperatures. The air dryer includes a temperature sensor associated with the valve block and a controller that can inhibit the operation of the valves when the temperature falls is a predetermined threshold until the heater can sufficiently warm the valve block. To prevent overheating of the heater, the controller can adjust the duty cycle of the heater as the inlet air temperatures increases.

What is claimed is: 
     
       1. An air dryer, comprising;
 an inlet for receiving a supply of compressed air; 
 a pair of inlet valves and a corresponding pair of exhaust valves positioned in a valve block for controlling the movement of the compressed air through a corresponding pair of desiccant towers; 
 a heater configured to warm the valve block; 
 a temperature sensor for outputting a signal indicating a temperature of the valve block; and 
 a controller interconnected to the inlet valves and outlet valves, the heater, and the temperature sensor, wherein the controller is programmed to determine whether the temperature of the valve block is below a first predetermined threshold and if so, inhibit the operation of the inlet valves and the exhaust valves until the temperature of the valve block is above a second predetermined threshold. 
 
     
     
       2. The air dryer of  claim 1 , wherein the controller is programmed to operate the heater when the temperature of the valve block is below the first predetermined threshold until the temperature of the valve block is above the second predetermined threshold. 
     
     
       3. The air dryer of  claim 2 , wherein the controller is programmed to only operate the inlet valves and the exhaust valves when the temperature of the valve block is above the second predetermined threshold. 
     
     
       4. The air dryer of  claim 3 , further comprising a second temperature sensor interconnected to the controller and positioned in the inlet for outputting a signal indicating a temperature of the compressed air in the inlet. 
     
     
       5. The air dryer of  claim 4 , wherein the controller is programmed to operate the heater according to the temperature of the compressed air in the inlet. 
     
     
       6. The air dryer of  claim 5 , wherein the controller is programmed to operate the heater according to a full duty cycle when the temperature of the compressed air in the inlet falls within a first range and according to a reduced duty cycle when the temperature of the compressed air in the inlet falls within a second range. 
     
     
       7. The air dryer of  claim 6 , wherein the controller is additionally programmed to operate the heater according to a normalized input voltage. 
     
     
       8. The air dryer of  claim 4 , wherein the controller is programmed to delay the opening of the exhaust valve that corresponds to the inlet valve being opened when switching movement of the compressed air between the pair of desiccant towers if the compressed air in the inlet is below a predetermined temperature. 
     
     
       9. A method of preventing frozen air dryer valves, comprising the steps of:
 providing an air dryer having an inlet for receiving a supply of compressed air, a pair of inlet valves and a corresponding pair of exhaust valves positioned in a valve block, a heater configured to warm the valve block, and a temperature sensor for outputting a signal indicating a temperature of the valve block; and 
 determining whether the temperature of the valve block is below a first predetermined threshold and if so, inhibiting the operation of the inlet valves and the exhaust valves until the temperature of the valve block is above a second predetermined threshold. 
 
     
     
       10. The method of  claim 9 , further comprising the step of operating the heater when the temperature of the valve block is below the first predetermined threshold until the temperature of the valve block is above the second predetermined threshold. 
     
     
       11. The method of  claim 10 , wherein the air dryer further comprises a second temperature sensor interconnected to the controller and wherein the step of operating the heater comprises operating the heater according to the temperature of the compressed air in the inlet. 
     
     
       12. The method of  claim 11 , wherein the step of operating the heater according to the temperature of the compressed air in the inlet comprises operating the heater with a full duty cycle when the temperature of the compressed air in the inlet falls within a first range and with a reduced duty cycle when the temperature of the compressed air in the inlet falls within a second range. 
     
     
       13. The method of  claim 12 , further comprising the step of operating the heater according to a normalized input voltage. 
     
     
       14. The method of  claim 9 , further comprising the step of delaying the opening of the exhaust valve that corresponds to the inlet valve being opened when switching movement of the compressed air between the pair of desiccant towers if the compressed air in the inlet is below a predetermined temperature.